CHAPTER VIII.

DETAILS OF PROTOZOA.

The Protozoa are divided into three orders.

I.--_Gregarinidæ._ II.--_Rhizopoda._ III.--_Infusoria._

I. The _Gregarinidæ_ consist of minute protozoa, parasitic in the interior of insects, &c., and like other internal parasites are colourless, as we should expect.

II. The _Rhizopoda_ may, for our purpose, be divided into the naked forms like _Amoeba_, and those which possess a skeleton, such as the Radiolaria, the Foraminifera and the Spongia.

Of these the naked forms are colourless, or uniformly tinted, excepting the flush already described as emphasizing the contractile vesicle.

The _Foraminifera_ are the earliest animals that possess a skeleton or shell, and though generally very small, this shell is often complex, and of extreme beauty, though their bodies retain the general simplicity of the protozoa, indeed, they are said to possess no contractile vesicle. Still the complexity of their shells places them on a higher level than the naked rhizopoda.

In these animals we find the first definite colour, not as a pattern, but as simple tinting of the protoplasm. The general hue is yellowish-brown (as in _Amoeba_), but deep red is not uncommon. The deepest colour is found in the oldest central chambers, becoming fainter towards the periphery, where it is often almost unrecognisable.[19]

The _Radiolaria_ are minute organisms with still more complex skeletons, and are considered by Haeckel[20] to be more highly organized than the preceding order. They consist of a central portion containing masses of minute cells, and an external portion containing yellow cells. Here we have the first differentiation of parts in the external coating and internal capsule, and side by side with this differentiation we find colour more pronounced, and even taking regional tints in certain forms.

We may notice the following genera as exhibiting fine colour:--

_Red._ Eucecryphalus, Arachnocorys, Eucrytidium, Dictyoceras.

_Yellow._ Carpocanium, Dictyophimus, Amphilonche.

_Purple._ Eucrytidium, Acanthostratus.

_Blue._ Cyrtidosphæra, Coelodendrum.

_Green._ Cladococeus, Amphilonche.

_Brown._ Acanthometra, Amphilonche.

Examples of these may be seen in the plates of Haeckel's fine work, and as an illustration of regional decoration we cite _Acanthostratus purpuraceus_, in which the central capsule is seen to run from red to orange, and the external parts to be colourless, with red markings in looped chains.

_Spongocyclia_ also exhibits this regional distinction of colour very clearly, the central capsule being red and the external portion yellow.

The _Spongida_, or sponges, are, broadly speaking, assemblages or colonies of amoeba-like individuals, united into a common society. Individually the component animals are low, very low, in type, but their union into colonies, and the necessity for a uniform or common government has given rise to peculiarities that in a certain sense raise them even above the complex radiolaria. Some, it is true, are naked, and do not possess the skeleton that supports the colony, which skeleton forms what we usually call the sponge; but even amongst these naked sponges the necessity for communal purposes over and above the mere wants of the individual, raises them a step higher in the animal series. A multitude of individuals united by a common membrane, living in the open sea, it must have happened that some in more immediate contact with the food-producing waters, would have thriven at the expense of those in the interior who could only obtain the nutriment that had passed unheeded by the peripheral animals. But just as in higher communities we have an inflowing system of water and an out-flowing system of effete sewerage quite uncontrolled, and, alas, generally quite unheeded by the individuals whose wants are so supplied; so in the sponges we have a system of inflowing food-bearing water and an out-flowing sewage, or exhausted-water system. This is brought about by a peculiar system of cilia-lined cells which, as it were, by their motion suck the water in, bringing with it the food, and an efferent system by which the exhausted liquid escapes. These cilia-lined cells are the first true organs that are to be found in the animal kingdom, and according to the views we hold, they ought to be emphasized with colour, even though their internal position renders the colouration less likely. This we find actually to be the case, and these flagellated cells, as they are called, are often the seat of vividest colour.

The animal matter, or sarcode, or protoplasm of sponges falls into three layers, just as we find the primitive embryo of the highest animals; and just as the middle membrane of a mammalian ovum develops into bone, muscle and nerve, so the middle membrane (mesosarc) of the sponges develops the hard skeleton, and in this most important part we find the colour cells prevail. Sollas, one of our best English authorities upon sponges, writes, "The colours of sponges, which are very various, are usually due to the presence of pigment granules, interbedded either in the _endosarc of the flagellated cells_, or in the mesodermic cells, usually of the skin only, but sometimes of the whole body."[21]

We can, then, appeal most confidently to the protozoa as illustrating the morphological character of colouration.

[19] Leidy. Rhizopoda of N. America, p. 16. [20] Haeckel. Die Radiolarien, Berlin, 1862. [21] Sollas. Spongidæ. Cassell's Nat. Hist. Vol. vi., p. 318.